Intermediate commit on refactoring 'validateAndExtract'
Lower version requirement for 'text' so that it compiles on nix
Fix compile error in 'validateAndExtractSf'
Reformat inherited pkgs in default.nix
Fill in implementation of 'validateAndExtractSeg'
Switch 'validateAndExtractSegItem' over to return DEValues
Add a somewhat hacky case for 'validateAndExtractSegItem', adjust test

While this passes the first test and works for basic cases, the implementation
is really a kind of a hack to reuse the previous cases. In any case a lot
more verification and tests are needed.